It can be easily observed that every annulet, for any x ∈ L, [x]∗ is a normal
ideal in L. We denote the set of all annulets of L by N0(L). That is, N0(L) =
{[x]∗|x ∈ L}. Annulets have many important properties. We give some of them in
the following lemma which can be easily verified.